1. Field of the Invention
The present invention relates to a continuously working press for manufacturing chipboard, fiberboard, plywood panels or the like, and more particularly, to a continuously working press having flexible, endless steel bands which transmit applied pressure and draw the pressing material through the press.
2. Discussion of the Related Art
Continuously working presses are well known with art. Such presses typically include flexible steel bands, which are guided around a press table and a press ram via drive drums and deflection drums and, with an adjustable press gap, are supported against the abutment of table and press ram via travelling roller bars. The roller bars are guided with their axes oriented transversely to the band running direction and are positively guided at the entry into the press zone by feed sprockets.
For example, German Offenlegungsschrift 3,117,778 discloses a continuous press in which the rolling bodies revolving between the abutments and the steel bands consist of individual roller bars spanning the entire width of the press area. These roller bars, which can be two to three meters long, depending on the width of the press area, have a diameter of D of 14 to 18 mm, preferably 14 to 16 mm, at a tolerance of 15 .mu.m and are guided free of cages or chains around the table and press ram. This dimensioning of the roller bars is based on the observation that the smaller the roller-bar diameter D--and thus the smaller the support spacing K--the greater the functional reliability of the rolling support of the roller bars relative to the steel band.
According to this dimensioning, the steel bands have a thickness d which approximately corresponds to the quotient D/10, the roller bars being fed into the press zone at a relative gap spacing s of approximately the thickness d of the steel bands. This dimensioning is intended to reduce the elastic deformation of the steel bands in the press zone. Deflection drums of unacceptably large diameter are avoided by selecting the steel-band thickness d to be below 1.8 mm.
In German Offenlegungsschrift 3,117,778, reference is made to the fact that congeneric devices having considerable elastic deformation (e.g. German Offenlegungsschrift 2,215,618), as well as slight elastic deformation (e.g. Swiss Patent Specification 327,433) have been disclosed which in each case involved considerable disadvantages, namely defective rectilinear exit of the roller bars. Based on these prior experiences, it was decided in German Offenlegungsschrift 3,117,778 to reduce the elastic deformation of the steel bands as far as possible. However, this is accompanied by the necessity, emphasized in German Offenlegungsschrift 3,117,778, of providing a very small production tolerance of 15 .mu.m for the roller bars, which, of course, results in the manufacture of the roller bars becoming considerably more expensive.
The continuous press disclosed in German Offenlegungsschrift 3,117,778 is explicitly designed to provide a cage-free and chain-free guidance of the roller bars. Guide chains of this type are disclosed, for example, by Swiss Patent Specification 327,433. If the roller bars do not run exactly rectilinearly in presses of this type, considerable stresses occur in the chains, so that the chains can be destroyed. Obviously, this exact rectilinear exiting of the roller bars also cannot be guaranteed in the continuous press according to German Offenlegungsschrift 3,117,778. Thus, its roller bars are intended to run free of cages or chains.
However, satisfactory operation of the press according to German Offenlegungsschrift 3,117,778 with a cage-free and chain-free guidance of the roller bars has proven impossible. During start-up and idling operation or when the press ram is lifted, the frictional connection of the sagging steel band on the roller bars is neutralized and an indeterminate roller-bar spacing develops in the guidance-free roller-bar system, i.e. a random relative arrangement of the individual roller bars results. In particular, restarting in load operation is not possible without wear and malfunction, including roller-bar fracture.
A further disadvantage of the known press is that the use of steel-band thicknesses of d=1.1 to 1.8 mm limits the applied pressure to be transformed in practice to a value which is inadequate for certain requirements, e.g. for presses of 40 bar applied pressure and at press lengths over 40 m for manufacturing highly compressed chipboard panels of highest quality.